Add type signature for a WSGI Application to wsgiref (#825)

This type is something core to Python and is useful when typing web applications,
but doesn't actually exist in the stdlib anywhere.  I put this in wsgiref, but I am
open to suggestions as for a better place.

(Original PR by @rowillia.)

stdlib/2/wsgiref/types.pyi

@@ -0,0 +1,34 @@
+# Type declaration for a WSGI Function in Python 2
+#
+# wsgiref/types.py doesn't exist and neither does WSGIApplication, it's a type
+# provided for type checking purposes.
+#
+# This means you cannot simply import wsgiref.types in your code. Instead,
+# use the `TYPE_CHECKING` flag from the typing module:
+#
+#   from typing import TYPE_CHECKING
+#
+#   if TYPE_CHECKING:
+#       from wsgiref.types import WSGIApplication
+#
+# This import is now only taken into account by the type checker. Consequently,
+# you need to use 'WSGIApplication' and not simply WSGIApplication when type
+# hinting your code.  Otherwise Python will raise NameErrors.
+
+from typing import Callable, Dict, Iterable, List, Optional, Tuple, Type, Union
+from types import TracebackType
+
+_exc_info = Tuple[Optional[Type[BaseException]],
+                  Optional[BaseException],
+                  Optional[TracebackType]]
+_Text = Union[unicode, str]
+WSGIApplication = Callable[
+    [
+        Dict[_Text, _Text],
+        Union[
+            Callable[[_Text, List[Tuple[_Text, _Text]]], Callable[[_Text], None]],
+            Callable[[_Text, List[Tuple[_Text, _Text]], _exc_info], Callable[[_Text], None]],
+        ]
+    ],
+    Iterable[_Text]
+]